1. Field of the Invention
The present invention relates to a switch having one first and at least one second external terminal as well as a temperature-dependent switching mechanism that makes, as a function of its temperature, an electrically conductive connection between the two external terminals for an electrical current to be conducted through the switch, the switching mechanism comprising a switching member which changes its geometrical shape between a closed position and an open position as a function of temperature and, in its closed position, carries the current flowing through the switch. The switch further comprises an actuating member which is permanently connected electrically and mechanically in series with the switching member.
2. Related Prior Art
A switch of this kind is known from U.S. Pat. No. 4,636,766 A.
The known switch comprises, as the switching member, a U-shaped bimetallic element having two legs of different lengths. Attached to the long leg is a movable contact element which coacts with a switch-mounted countercontact that in turn is connected in electrically conductive fashion to one of the two external terminals.
The shorter leg of the U-shaped bimetallic element is attached to the free end of an actuating member configured as a lever arm, which at its other end is joined immovably to the housing and is connected in electrically conductive fashion to the other of the two external terminals. The actuating member is a further bimetallic element which is matched to the U-shaped bimetallic element in such a way that when temperature changes occur, the two bimetallic elements deform in opposite directions and thus maintain the contact pressure between the movable contact element and the housing-mounted countercontact.
This switch is intended as an interrupter for high currents, which cause considerable heating of the bimetallic element through which current is passing, thus ultimately lifting the movable contact element away from the fixed countercontact. Ambient temperature influences are compensated for, in this context, by the aforementioned opposite-direction deformation of the bimetallic elements.
The principal disadvantage of this design is that two bimetallic elements are required, the temperature characteristics of which must be exactly matched to one another; this is physically complex and cost-intensive to implement. In order to compensate for production tolerances, the known switch is moreover mechanically adjusted after assembly, which constitutes a further disadvantage.
Since the two bimetallic elements are of geometrically very different design, they also have different long-term stabilities, so that readjustment would in fact be necessary from time to time. This is, however, no longer possible during use, so that long-term stability and thus functional reliability generally leave much to be desired.
A further disadvantage of this design consists in the large overall height resulting from the U-shaped bimetallic element.
The known current-dependent switch is thus of complex design, expensive, and not very reliable.
A further current-dependent switch known from EP 0 103 792 B1 has as the switching member a bimetallic spring tongue which is attached to the one external terminal and at its free end carries a movable contact element which coacts with a countercontact that is arranged at the free end of an elongated spring element that is attached at the other end to the other external terminal. The switch is connected with its external terminals in series with an electrical device in such a way that the operating current of that switch flows through the bimetallic spring tongue. As a rule, the known switch is moreover thermally coupled to the electrical device, so that it can follow its temperature changes.
If the temperature of the device now rises above an impermissible value, the bimetallic spring tongue lifts the movable contact away from the countercontact, thus interrupting the flow of current and preventing the electrical device from heating up further. The bimetallic spring tongue can also, however, be brought into this open position by an increased flow of current, since the bimetallic spring tongue heats up due to the electrical current flowing through it. The electrical properties of the bimetallic spring tongue can be set, in coordination with the mechanical properties and the kickover temperature, in such a way that it is in its closed position, in which it conducts the operating current of the electrical device, when the ambient temperature is below the switching temperature and the operating current is also below a response current intensity. If the operating current then rises above the permissible value, the bimetallic spring tongue heats up very rapidly and reaches its kickover temperature, whereupon it transitions into its open position.
This switch thus offers protection from both overtemperature and overcurrent.
Because of the elastic mounting of the countercontact, the contact and countercontact rub against one another during switching operations, so that contaminants and deposits are rubbed off the contact surfaces, ensuring a low contact resistance and thus a good electrical connection. The elastic mounting of the countercontact furthermore ensures low mechanical loading of the bimetallic spring tongue, since the countercontact yields to a limited extent. This prevents irreversible deformations of the bimetallic spring tongue. Since mechanical deformations of this kind can lead to a shift in the switching temperature, the overall result of this arrangement is to ensure high operating reliability.
A disadvantage with this known switch, however, is that because of the elastic deflection of the countercontact and the kickover of the bimetallic spring tongue into the open position, it requires a relatively large amount of space for the switching function of the temperature-dependent switching mechanism. A further disadvantage is the fact that during the transition from the closed position into the open position, the bimetallic spring tongue--like all bimetallic elements--passes through a so-called "creep" phase in which the bimetallic element deforms in creeping fashion as a result of a rise or drop in temperature, but does not snap over from its, for example, convex low-temperature position directly into its concave high-temperature position. This creep phase occurs each time the temperature of the bimetallic element approaches the kickover temperature from either above or below, and leads to appreciable changes in conformation. The creep characteristics of a bimetallic element can moreover also change even further as a result, particularly, of aging or long-term operation.
During the opening movement, creep can cause the pressure of the contact against the countercontact to weaken, thus leading to undefined switching states. During the closing movement, the contact can gradually approach the countercontact during the creep phase, thus possibly creating the risk of arcing.
These problems associated with the creep behavior of a bimetallic element are solved, in the case of a current-dependent switch as described in the aforementioned U.S. Pat. No. 4,636,766, in U.S. Pat. No. 4,389,630, or in EP 0 103 792, by the fact that the bimetallic spring tongue is equipped with dimples which do not suppress the creep phase completely, but do suppress it for the most part. These dimples or other mechanical actions upon the bimetallic element are complex and expensive features which moreover greatly reduce the service life of these bimetallic elements. A further disadvantage of the requisite dimple may be seen in the fact that not only different material compositions and thicknesses, but also different dimples, must be used for various performance classes and response temperatures.
In all the switches known from the prior art described so far, the creep phase is thus kept as short as possible, increasing or compensating pressure as well as additional dished portions being used for the purpose.
In all the switches described so far it is further felt to be a disadvantage that they will close again, even after having been strongly overheated, when the temperature drops again below the switching temperature. Such re-closing is in part prevented, according to the prior art, by the fact that a heating resistor is connected in parallel to the switching mechanism, which heating resistor carries a residual current when the switch is in its open position so that it will heat up so far that the bimetallic element remains above its response temperature. This function is known as self-holding effect. However, when the operating current is switched off completely, then such a switch will of course also cool down and go into its closed condition.
More recent safety demands require, however, that when a safety temperature above the snap-over temperature is exceeded, the switch should remain permanently open, regardless of any residual current.
Generally, a switch of this kind has been known from U.S. Pat. No. 4,885,560 A1 which describes a current-dependent switch comprising a bimetallic snap disk that carries two movable contacts each coacting with a fixed countercontact. Below its switching temperature, the bimetallic snap disk thus connects two external terminals that are connected with the fixed countercontacts. When the bimetallic snap disk heats up above is switching temperature, due to an excessively high current, then it lifts both movable contacts off the fixed countercontacts thereby breaking the circuit.
The bimetallic snap disk is fitted in this case centrally on an adjusting screw, and is pressed by a compression spring against the head of the adjusting screw. The head is fixed on the adjusting screw as such by means of fusible solder which will liquefy when a given safety temperature above the normal response temperature of the bimetallic snap disk is exceeded, whereupon the compression spring will urge the bimetallic snap disk away from the adjusting screw whereby the switch is opened in irreversible fashion.
Compared with the other switches discussed so far, this switch provides the advantage that an additional safety mechanism becomes active in the event an overheating condition should occur due to a high current flow with the result that the movable contacts get welded to the fixed countercontacts. If such welding should occur, the displacing force of the bimetallic snap disk would no longer be sufficient to lift the movable contacts off the fixed countercontacts, whereas the pressure of the compression spring still would be, because once the fusible solder has liquefied, there would be no other counteracting force.
The compression spring used must be very strong to ensure that the welded contacts will be safely re-opened. In normal operation, the high force of the compression spring acts centrally upon the bimetallic snap disk, the other side of which rests against the head of the adjusting screw, which is secured in its position by fusible solder. So, a very high mechanical pressure is exerted upon the center of the bimetallic snap disk, which has a negative effect on the service life and the reproducibility of the switch point.
A further disadvantage lies in the fact that in order to prevent contact blinking, the bimetallic snap disk must be provided with deep dimples so as to suppress the creeping phase.